A student says, 'The thermodynamic scale is defined by the expansion of mercury, so changing to an alcohol thermometer changes the thermodynamic temperature.' Explain both errors and how two such instruments can be used to measure the same bath temperature.

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Thermodynamic temperature is not defined by any particular material property. Changing instruments cannot change an unchanged bath's temperature; each instrument must be calibrated and allowed to equilibrate. Thermodynamic temperature is independent of the property of any particular substance. Mercury and alcohol expansion are thermometric responses used to measure the scale, not definitions of the scale. Calibrate each response against known thermodynamic temperatures, allowing nonlinear calibration if required. Place each in adequate thermal contact and wait for equilibrium; discrepancies then indicate measurement/calibration limitations.

One temperature, different instruments

A student says, 'The thermodynamic scale is defined by the expansion of mercury, so changing to an alcohol thermometer changes the thermodynamic temperature.' Explain both errors and how two such instruments can be used to measure the same bath temperature.

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